It is conventional practice, in the drilling and completion of wells, that prior to the placement of cement, the casing and hole are filled with drilling mud. In order to reduce contamination of the interface between the mud and the cement a bottom cementing plug is placed in the casing and pumped ahead of the cement slurry. The bottom plug is typically constructed with a one piece hollow metallic or a one piece non-metallic core having an elastomer covering molded to the core. The elastomer cover typically incorporates a plurality of wipers. The function of the wipers is to wipe the internal surface of the casing, maintain the separation of fluids during the displacement of the cement slurry down the casing and provide a means of sealing upon displacement of the plug. The bottom plug incorporates a rupture diaphragm that will rupture upon the bottom plug reaching a float shoe, float collar or landing collar located near or at the bottom of the casing. When the bottom plug reaches the float shoe, float collar or landing collar, the increase in fluid pressure will result in the diaphragm rupturing allowing the cement slurry to pass though the bottom plug and continue out the bottom of the casing, filling the space between the casing and the well bore.
Upon placing all the cement into the casing a top plug is located on top of the cement. The top plug is typically constructed having a solid elastomer, one piece metallic or one piece non-metallic core having an elastomer covering molded to the core, the elastomer cover incorporating a plurality of wipers. The function of the wipers is to wipe the internal surface of the casing, maintain the separation of fluid during the displacement of cement slurry down the casing and provide a method of providing a sealing mechanism upon landing the top plug on top of the bottom plug. When displacement of the cement slurry is complete the top plug will land on top of the bottom plug remaining in this position. The required length for the top plug and the bottom plug varies with the intended application, and is based upon a number of factors including the pressure being contained.
After the cement slurry has become hard, the top and bottom plugs are drilled out using a drilling bit. Typical prior art cementing plug sets incorporate teeth, lugs or threads that act to prevent the plugs from rotating during the drill-out procedure, thereby reducing the time required to perform the drill-out operation.